Microstructure-performance relationships in gas tungsten arc welded Hastelloy X nickel-based superalloy

This study aims at understanding the metallurgical and mechanical response of Hastelloy X nickel-based superalloy to the thermal cycle of gas tungsten arc welding (GTAW), post welding solution treatment (ST), and aging. In the as-weld condition, the weldment was characterized by the formation of the M6C carbide, sigma phase, and micro-solidification cracks in the interdendritic zones of the weld metal, and constitutional liquation of M6C carbide in the heat-affected zone. It is demonstrated that neither the weld metal nor the heat-affected zone could adversely affect mechanical properties of the welded joint in as-weld condition. However, to alleviate the microstructural heterogeneity, tensile residual stresses, and non-equilibrium microstructure developed due to the welding thermal cycle, the weldment was subjected to the solution treatment cycle (1177 degrees C/5 min) as post-weld heat treatment. The post-weld heat treatment enabled complete elimination of sigma phase. The tensile and creep properties of the weldment after the solution treatment cycle were compared to that of the asweld condition. Despite the 8% decrease in the strength of the weldment after the ST cycle, the sample not only still met the minimum mechanical requirements but also benefited from a homogenized and nearly equilibrium microstructure. Microstructural observations of the weldment after the aging cycle (800 degrees C/8h), as well as its mechanical response, revealed that the formation of continuous Cr-rich carbides at the grain boundaries of the base metal deteriorates the room temperature ductility and creep life of the joints.